AN1895 Datasheet, PDF (3/16 Pages) STMicroelectronics – EVAL6562-375W Evaluation Board

AN1895
1
Board description
Board description
The EVAL6562-375W evaluation board includes a Power Factor Correction (PFC) pre-
regulator for a 300W ATX 12V power supply unit (PSU). It is able to deliver 375W continuous
power on a regulated 400V rail from a wide range of mains voltage. This rail will be the input
for the cascaded isolated DC-DC converter (typically a forward converter) that will provide
the output rails of the silver box. Although the ATX specification envisages air cooling,
typically realized with a fan capable of an airflow in the range of 25-35 CFM, this is not
allowed for in the design of this evaluation board. Enough heat sinking will be provided to
allow full-load operation in still air. With an appropriate airflow and without any change in the
circuit, the evaluation board can easily deliver up to 400-420W.
The L6562 controller chip is designed for Transition-Mode (TM) operation where the boost
inductor works next to the boundary between Continuous (CCM) and Discontinuous
Conduction Mode (DCM). However, with a slightly different usage, the chip can operate so
that the boost inductor works in CCM, hence surpassing the limitations of TM operation in
terms of power handling capability. The gate-drive capability of the L6562 (±0.8A min.) is
also adequate to drive the MOSFETs used at higher power levels.
This approach, which couples the simplicity and cost-effectiveness of TM operation with the
high-current capability of CCM operation, is the Fixed-Off-Time (FOT) control. The control
modulates the ON-time of the power switch, while its OFF-time is kept constant. More
precisely, it will be used the Line-Modulated FOT (LM-FOT) where the OFF-time of the
power switch is not rigorously constant but is modulated by the instantaneous mains
voltage. Please refer to [2] for a detailed description of this technique.
Table 1 summarizes the electrical specification of the application and Table 3 lists
transformer specifications.
The electrical schematic is shown in Figure 1 and the PCB layout in Figure 2.
Appendix A lists the bill of materials.
Table 1. Electrical specifications
Parameter
Line voltage range
Minimum line frequency (f_L)
Regulated output voltage
Rated output power
Maximum 2f_L output voltage ripple
Hold-up time
Maximum switching frequency (@ V_IN = 90 V_AC, P_OUT = 375 W)
Minimum estimated efficiency (@ V_IN = 90 V_AC, P_OUT = 375W)
Maximum ambient temperature
Value
90 to 265 V_AC
47 Hz
400 V
375 W
20V pk-pk
17 ms
100 kHz
90%
50° C
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